The field of this invention of that of umbilical reels which store and handle hose and/or electric and/or fiber optic control lines for deepwater offshore service. These reels typically pay out these lines, called umbilicals, and mechanics clamp the umbilical to a drilling riser or other pipe string being run to the seafloor. The actual weight of the umbilical is typically supported once it leaves the reel and in the water by the riser or pipe to which it is clamped. Typically these reel units have to be closely monitored to insure that excessive tension which can destroy the umbilical is not encountered as it is being deployed or in the event of unexpected movement of the riser or pipe to which it is clamped.
When the drilling riser or other pipe string is lowered, an operator will rotate the spool to allow umbilical to be paid off in accordance with the downward movement of the riser or pipe. In some cases, the motor can be left in the take up mode, and the umbilical simply be pulled off the spool against a relatively constant torque provided by the motor power.
The spool portion of a reel can typically be locked into position by the brakes, the motor, or a manual locking pin.
A danger to the umbilical or reel can occur in the event that the drilling riser or other pipe string to which the umbilical is attached is lowered while the reel spool is locked in position. The reel spool can be locked in position because someone forgot to release the locking pin, the brakes are set, or the motor is locked. When this happens, an umbilical worth hundreds of thousands of dollars can be destroyed by the excessive tension and personnel can be injured.
Alternately, if the riser and umbilical are being deployed and the air pressure which runs the reel is lost, the failsafe brakes will automatically lock creating a chance that excessive tension will destroy the umbilical before the condition is recognized.
A slip clutch has sometimes been added to the reel to allow the spool to turn when a relatively fixed preset tension limit on the umbilical is exceeded. The spool holds several layers of umbilical and as successive layers of umbilical are deployed the spooling diameter decreases and this effect causes the tension on the umbilical to increase because the preset torque remains fixed. As offshore services are required in deeper water the length of umbilical deployed from the reels increases and this causes the diameter differential between a fully loaded spool and a fully deployed spool to increase thus multiplying the maximum tension on the umbilical during deployment. A fixed preset torque which provides a tension of 1,000 pounds on the umbilical when the spool is fully loaded can vary to over 3,000 pounds when the spool is nearly empty.